The instant invention relates to extrusion systems, and more particularly to a crosshead die for a multiple-layer extrusion system.
In the manufacture of plastic-coated wire, rubber-coated wire, plastic tubing, or rubber tubing, molten plastic and/or rubber is extruded by means of a crosshead extrusion system which receives a stream of molten material and causes the molten material to be distributed around the circumference of a wire or tube. In this connection, a variety of crosshead devices have heretofore been known in the art. For example, U.S. Pat. No. 5,183,669 to Guillemette discloses a crosshead extruder system which is operative for applying a single layer of plastic onto a wire. It is also known in the art to simultaneously extrude more than one layer of plastic and/or rubber. Such a process is accomplished by means of multi-layer crosshead devices such as illustrated in the U.S. Pat. No. 5,108,683, to Anand and U.S. Pat. No. 4,798,526 to Briggs et al. These devices utilize one or more crosshead dies which are sequentially mounted onto an extruder for co-extrusion of multiple layers. In this regard, it is well known in the crosshead art that it is extremely difficult to establish an even and balanced flow of molten plastic or rubber around the circumference of a wire or tube. For example, it is known that sharp turns in the material flow paths create "dead-spots" in the crosshead in which plastic or rubber material stagnates and becomes overheated. When plastic or rubber overheats or "cooks", it turns black, and thereafter, when the burnt material makes its way back into the flow stream, it causes discolored specs in the coating. It is also known that splitting and reblending the material stream causes weld or joint lines wherein the extruded material is not evenly blended together. These flow problems are of particular interest in multiple-layer co-extrusion devices because there are multiple crosshead dies and multiple flow-paths for the layers of material being extruded. In many of the known crosshead dies, the balanced flow problems are exaggerated even further because of lengthened flow paths, and pressure drops in enlarged flow areas.
The instant invention provides a crosshead die for an extrusion system which achieves a completely uniform and balanced flow with no "dead spots", so that a consistent, even thickness of extruded material is distributed therearound. Briefly, the instant crosshead die comprises a cylindrical body having a first and second ends, a reduced diameter end portion at the second end thereof, and an axial bore which extends through the end portion. The cylindrical body further has a recessed surface in the first end thereof, and a circular groove in the recessed surface. The groove has an inner edge and an outer edge, and the outer edge includes two inwardly curved blending wedges which are positioned in opposed relation on a vertical centerline. The inner edge of the groove merges into a frusto-conical inner wall which tapers axially inwardly and merges with the axial bore in the reduced diameter end portion. The inner edge of the groove has a lesser height than the recessed surface. The cylindrical body further includes a concave flow inlet on the outer wall which is aligned with the vertical centerline. The body still further includes first and second symmetrical flow channels which extend into the body from the flow inlet and diverge outwardly from the vertical centerline toward the outer wall and then turn forwardly and open outwardly into the groove. The openings of the flow channels in the flow inlet are adjacent and they cooperate to form a natural wedge-shaped flow splitter in the flow inlet.
A second embodiment of the crosshead die is operative for use with large diameter wire or tubing wherein the axial bore of the die has a larger diameter. In this connection, it can be seen that the frusto-conical inner wall also has a larger end diameter. To accommodate the larger diameter bore and the wider taper of the inner wall, the flow inlet of the die is extended around the outer wall for approximately 10 degrees on each side of the vertical centerline and the flow channels extend into the body from the outer ends of the flow inlet. The spaced positions of the flow channels allow the channels to pass through the body without intersecting the inner frusto-conical wall. Since the openings of the flow channels are in spaced relation, a wedge-shaped flow splitter is provided in the flow inlet for dividing the flow evenly between the two flow channels.
A third embodiment of the crosshead die eliminates the reduced diameter end portion and incorporates a second groove in the second end surface for applying another layer of material. The inlet for second groove is positioned on the vertical centerline at the bottom of the die opposite the first inlet. The flow channels for the second groove diverge outwardly and then turn rearwardly to open into the second groove. The inner edge of the second groove merges into an outwardly tapered frusto-conical wall.
Accordingly, it is an object of the instant invention to provide a crosshead die which has a balanced and even flow of material without any dead spots.
It is another object to provide a flow splitter at the flow inlet of a crosshead die.
It is yet another object to provide wedge-shaped flow blenders in a circular flow groove of a crosshead die.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.